Various ultrasonic methods currently are employed in diagnostic investigations of living tissue. One such widely used method is the sonar-like A-scan method wherein short ultrasonic pulses from an ultrasonic transducer coupled to the patient's skin are projected into the tissue. Reflections from tissue interfaces received by the transducer are detected and the resultant electrical signal may be displayed at the screen of a cathode ray tube. Since the round trip travel time of the reflected pulse is related to the distance from the transducer to the reflecting layer, the presentation of these waves at the cathode ray tube permits the operator to directly measure such distances.
The well-known B-scan method is similar to the above-described A-scan method in that both methods employ a narrow-beam transducer to project short ultrasonic pulses into the tissue and to detect pulses reflected from tissue interfaces. However, with B-scan, a two-dimensional image is produced by scanning the ultrasonic beam, either electronically or mechanically, across the area of interest. A cathode ray tube display may be provided in which one of the orthogonal deflection voltages is proportional to the transducer position and the other to the time elapsed since the last pulse was transmitted. The reflected ultrasonic pulse is used to intensity modulate the display.
Often, the transducer means employed with such A and B-scan systems is located within a container of acoustic transmission liquid, which container has a liquid tight acoustically transparent window therein for acoustically coupling to the skin of the subject under investigation. A coupling liquid such as water, within which the velocity of propagation of acoustic waves is substantially the same as in the tissue, often is employed. Refractive displacement of acoustic waves travelling across the window interfaces is minimized by use of a thin window. A fluid path length within the water equal to or greater than the greatest imaging depth within the tissue is required to prevent multiple reflections between the transducer and transducer window from being superimposed upon the image display.